Positional control for an electric motor in a servo system is generally executed by a control system based on a cascade structure in which a positional control loop is a main loop and a speed control loop and a current control loop are minor loops.
For detecting a rotational speed of an electric motor in the positional control as described above, generally a pulse encoder attached to the electric motor is used and a rotational speed is obtained by dividing a number of output pulses generated within a specified sampling period of time by sampling time.
FIG. 41A and FIG. 41B show a situation where a rotational speed detection is executed in an extremely low rotational speed area in a case in which the speed detection method described above is used. It is assumed that the electric motor rotates at a constant speed. FIG. 41A shows a timing of pulse output from an encoder as well as a timing for sampling, while FIG. 41B shows a rotational speed of the electric motor detected according to an output from the encoder and the actual rotational speed.
In an extremely low rotational speed area, as shown in FIG. 41A, as a state where a number of pulses inputted from the pulse encoder within a sampling period is zero is frequently generated, the detected rotational speed is a pulse-like one as shown in FIG. 41B. In a state where rotation of the electric motor is down, the frequency of generation of pulses becomes further lower.
Because the detected rotational speed is a pulse-like one as described above, also an output torque from the electric motor, when a rotational speed control is executed in an extremely low rotational speed area or in a stopped state, becomes pulse-like, and if the gain is made larger, strength of the pulse-like torque also becomes larger, which causes a minute vibration.
It should be noted that the stopped state as defined herein means that positional displacement of an electric motor does from a position decided according to a stop command value within a range specified according to a preset precision, and may be expressed as a stop-instructed state.
As a method of controlling the minute vibration in a stop-instructed state caused due to a particular rotational speed detecting system with a pulse encoder, there has been known a method in which a gain of a speed control loop is made smaller in a stop-instructed state such as a servo lock state, or a method in which a ripple factor included in a detected rotational speed value is made smaller by making larger a time constant of a filter inserted into a speed feedback in a stop-instructed state.
FIG. 42 shows a position control unit for an electric motor disclosed in Japanese Patent Laid-Open No. 245312/1987. This position control unit comprises a position detecting circuit 202 for counting a number of pulses outputted from a pulse encoder 201 attached to an electric motor (servo motor) 1 and outputting a position signal .theta.m for the electric motor 1; a speed detecting circuit 203 for computing a rotational speed of the electric motor 1 depending on a number of pulses received within a prespecified sampling period of time according to the method as described above and outputting a speed signal .omega.m for the electric motor 1; a position control circuit 204; a speed control circuit 205; a current control circuit 206; a gains switching circuit 207; and a current detector 208. It should be noted that a load machine 4 is connected via a torque transmission mechanism 3 to the electric motor 1.
The speed control circuit 205, current control circuit 206, and gain switching circuit 207 are connected to each other through a cascade structure; the position control circuit 204 computes a speed command so that a deviation between a positional command signal .theta.m* given from outside by, for instance, a positional command signal generating circuit and a positional command .theta.m for the electric motor 1 will become smaller and outputs a speed command signal .omega.m*, the speed control circuit 205 computes a current command so that a deviation between the speed command signal .omega.m* and the speed signal .omega.m for the electric motor 1 will become smaller and outputting the current command signal I*; and the current control circuit 206 provides controls so that a current I in the electric motor 1 detected by the current detector 8 will follow the current command signal I*.
The gain switching circuit 207 switches a gain of the speed control circuit 205 to a value smaller than a normal value when it is determined that the electric motor 1 has reached a target value for positional control.
In this position control unit, at a point of time when the electric motor 1 has reached a target value for positional control, namely at a point of time when the stop-instructed state has been effected, the gain switching circuit 207 switches a gain of the speed control circuit 205 to a value smaller than the normal value. With this mechanism, amplitude of a pulse-like current command signal I*, as shown in FIG. 41, outputted from the speed control circuit 205 according to a pulse-like detected rotational speed is suppressed, and generation of a minute vibration is suppressed.
FIG. 43 shows a position control unit for an electric motor disclosed in Japanese Patent Laid-Open No. 308788/1993. In FIG. 43, the same reference numerals are assigned to sections corresponding to those in FIG. 42, and description thereof is omitted herein.
In this position control unit, there are provided a low-pass filter 209 which filters a speed signal .omega.m from the speed detecting circuit 203, and a time constant switching circuit 210 which switches a time constant for the low-pass filter 209 to a value larger than the normal value when it is determined that the electric motor 1 has reached a target value for positional control.
In this position control unit, at a point of time when the electric motor 1 has reached a target value for positional control, namely at a point of time when the stop-instructed state has been effected, the time constant switching circuit 210 switches a time constant for the low-pass filter 209 to a value larger than the normal value, when a cut-off frequency of the low-pass filter 209 becomes lower than the normal value and also a waveform of the detected speed signal .omega.m having passed through the low-pass filter becomes smoother, and as a result also a waveform of the current command signal I* outputted from the speed control circuit 205 becomes smoother with generation of the minute vibration described above suppressed.
However, in a case where rigidity of the torque transmission mechanism 3 is low and mechanical resonance between the electric motor 1 and the load machine 4 is generated, sometimes the minute vibration in the stop-instructed state can not be suppressed in the position control unit as described above. A description is made below for the reason.
At first, description is made for the current control system comprising the current control circuit 206 and the current detector 208. The current detecting value I outputted from the current detector 208 generally includes, in addition to a current Im factor for the electric motor 1, a noise factor (described as In hereinafter). For this reason, even when a value of the current command signal I* is zero, the current control circuit 206 tries to provide controls so that the current Im in the electric motor 1 will follow the noise factor In, and for this reason a frequency factor similar to the noise factor In is included in a torque of the electric motor 1.
This noise factor In often shows the characteristics of white noise, and contains a excitation factor for a given mechanical resonance frequency, so that a phenomenon of a minute vibration having a vibration frequency closer to the mechanical resonance frequency is generated in the stop-instructed state. Especially, in a case where an amplitude of a minute vibration in the stop-instructed state is smaller than a 1 pulse factor from the encoder 201, an output from the position detecting circuit 202 or from the speed detecting circuit 203 is zero, so that suppression of this minute vibration is impossible.
As described above, the phenomenon of a minute vibration generated due to a noise factor is generated in an area not under control by the speed control system, so that, in the conventional type of position control unit in which control for suppressing the minute vibration is provided by the speed control system, suppression of the minute vibration is impossible.
As described above, in the conventional type of position control unit, in a case where there is a mechanical resonance between an electric motor and a load machine, it is impossible to suppress a minute vibration in the stop-instructed state generated due to a noise factor in a current control circuit, in other words, in a torque control means, which disables high precision positional control.